The present invention relates generally to methods for the synthesis of optically pure 4-alkenyl- or 4-alkanyl-2-hydroxytetronic acid aci-reductone compounds.
The aci-reductone, 4-(4-chlorophenyl)-2-hydroxytetronic acid compound (CHTA), possesses antilipidemic and antiaggregatory properties which differ from those of the classical phenoxyacetic acids as has been disclosed in Witiak et al. J. Med. Chem., 1988, 31:1437-1445 and Kamanna et al., Lipids, 1989, 24:25-32. Although unsubstituted-, 2-alkanyl- and 2-acyltetronic acids are frequently found in nature, the 2-hydroxy substituted redox system is found only in vitamin C and its closely related relatives (isoascorbic acid, erythroascorbic acid) and derivatives, and the macrolide antibiotic chlorothricin.
The antiaggregatory activities of 2-hydroxytetronic acid aci-reductone compounds such as CHTA are of interest since blood platelets are involved in the genesis of atherosclerosis. 2-Hydroxytetronic acid aci-reductones inhibit cyclooxygenase (CO)-induced human platelet aggregation and secretion of [.sup.14 C]-serotonin in a concentration-dependent manner at equivalent doses, as reported in Witiak et al., J. Med. Chem., 1982, 25:90-93.
The CHTA compound inhibits platelet function by both enzyme inhibitory and ROS scavenging mechanisms resulting ultimately in the blockage of thromboxane A.sub.2 synthesis. Redox analogues, such as 2-hydroxytetronic acid, may function as antioxidants in membranes and interfere with free radical processes involved in the biosynthetic elaboration of cyclic prostaglandin endoperoxides (PGG.sub.2 and PGH.sub.2), and, subsequently, thromboxane A.sub.2 from arachidonic acid.
aci-Reductones, such as CHTA, possess numerous biological properties and have potentially many therapeutic applications. They exhibit antilipidemic and antithrombotic activities and increase the effectiveness of IL-2 promoted lymphokine-activated killer (LAK) cell activity in human peripheral blood mononuclear cells (PBMC).
aci-Reductones inhibit CO-dependent AA-induced platelet aggregation. See Witiak et al., J. Med. Chem., 1982, 25:90-93; Witiak et al., 1988, J. Med. Chem., 31:1437-1445 and Witiak et al., J. Med. Chem., 1986, 29:2170-2174. A positive linear free energy relationship is observed between enzyme inhibition and calculated hydrophobicity (.pi.) parameters. Thus, 4-biphenyl and 4-(4'-chlorobiphenyl)-2-hydroxytetronic acids possess an estimated `.pi.` of 1.96 and 2.67 and inhibit AA-induced platelet aggregation with IC.sub.50 s of 135 and 44 .mu.M, respectively.
4-Aryl-2-hydroxytetronic acids potentiate IL-2-induced LAK activity. This activity is related in part to CO inhibition. Highly tumoricidal lymphocytes induced by IL-2 have therapeutic potential in the treatment of cancers for which conventional antineoplastic therapy is not useful. The by-products of IL-2 activation, PGE.sub.2 and reactive oxygen species (ROS) such as superoxide anion radical, abrogate LAK activity. aci-Reductones such as 4-aryl-, 4-alkanyl- and 4,4-spiroalkanyl-2-hydroxytetronic acids inhibit CO and the production of PGE.sub.2 as well as scavenge ROS, thus improving IL-2-induced LAK activity. In standard 4-hour .sup.51 Cr release assays, the improvement in LAK activity observed is comparable to the combined synergy obtained using the CO inhibitor indomethacin and ROS scavenging enzymes superoxide dismutase (SOD) and catalase. See, Triozzi et al., Int. J. Immunopharmac., 1993, 15:47-54 and Witiak et al., Am. Canc. Res. Mtg., 1993, Florida. Thus, aci-reductones may be useful in potentiating IL-2 cancer therapy.
aci-Reductones are also antilipidemic and lower total serum cholesterol, triglycerides, VLDL, and LDL in cholesterol/cholic acid-fed rats. These compounds have been found to decrease apoB in VLDL in vivo and inhibit copper-catalyzed LDL oxidation in vitro. See, Witiak et al., J. Med. Chem., 1982, 25:90-93, (1982); Witiak et al., Actual Chem. Ther., 1988, 15:41-62 (1988) and Witiak et al., J. Med. Chem., 1988, 31:1437-1445.
Free-radicals play a significant role in UV-, drug- and xenobiotic-induced toxicities and activate molecular oxygen to superoxide and other ROS including hydrogen peroxide and hydroxyl radical. Defense mechanisms, including enzymes such as SOD and catalase and radical scavengers such as glutathione, retinoic acid and ascorbic acid protect proteins and nucleic acids from free-radical toxicities by quenching ROS. Inadequate protection from ROS results in myocardial ischemia, photosensitivity, radiation sensitization, red cell hemolysis and atherosclerosis. 4-Aryl-2-hydroxytetronic acids have been found to possess antioxidant efficiencies similar to probucol and .alpha.-tocopherol. See Witiak et al., "Trends in Medicinal Chemistry", pp. 243-256, Blackwell Scientific Publications: Oxford, 1990.
Syntheses for 2-hydroxytetronic acids other than ascorbic acid have been reviewed by Haynes and Plimmer in "Tetronic Acids," Quart. Rev., 1960, 292-315, and by Shank, "Reductones," Synthesis, 1972, 176-190. 2-Hydroxytetronic acids have generally been prepared using three different routes: (1) hydroxyl group insertion at the 2 position of the corresponding tetronic acid nucleus; (2) intramolecular Claisen cyclization of substituted glyoxylate esters; and (3) base-promoted cyclization of 2,4-dihydroxy-3-ketobutanoates.
Witiak and Tehim, J. Org. Chem., 1987, 52:2324-2327 synthesized the 5- and 6-membered spiro 2-hydroxytetronic acids using propargyl alcohol conversion to methyl spirotetronates by treatment with sodium methoxide. Attempted hydroxylation at the 2-position by .alpha.-lithiation and reaction with dibenzoylperoxide provided only a 6% yield of the corresponding 2-benzoyloxytetronic acid. However, the 2-hydroxyl group was introduced in good yields by lithiation using lithium diisopropylamide (LDA), boronate ester formation [B(MeO).sub.3 ] and oxidative hydrolysis (AcOH, H.sub.2 O.sub.2). Methyl 2-hydroxytetronate was converted to the corresponding aci-reductone by stirring in 48% HBr at 45.degree. C. for 12 hours. Ireland and Thompson, J. Org. Chem., 1979, 44:3041-3052, utilized the Claisen condensation for construction of 2-hydroxytetronic acids.
Witiak and Tehim, J. Org. Chem., 1987, 52:2324-2327 prepared 5- and 6-membered spiro-2-hydroxytetronic acids using strategies developed by Ireland and Thompson, supra. This method was superior to use of hydroxyl group insertion methods because fewer steps were necessary and overall yields were higher. For example, intramolecular Claisen cyclization of easily prepared methoxy or benzyloxy thiocarboxylate intermediates using LDA or lithium hexamethyldisilazide (LiHMDA) at -78.degree. C. occurred in high yields. The resultant 2-methoxytetronic acids underwent deprotection by acetylation and subsequent reaction with BBr.sub.3, whereas the 2-benzyloxytetronic acids were convertible to target 2-hydroxytetronic acids by transfer hydrogenation.
Witiak and Tehim, J. Org. Chem., 1990, 55:1112-1114 developed the first synthesis for optically pure (S)-(+)-4-phenyl-2-hydroxytetronic acid using the Claisen cyclization under kinetically controlled conditions. The 2-benzyloxymethoxyacetate derivative of the corresponding methyl mandelate underwent such cyclization at -100.degree. C. using the sterically hindered non-nucleophilic base, lithium dicyclohexylamide (LiDCyA). Subsequent benzyl group deprotection of the tetronic acid generated the desired compound in low overall yields; 12% for both steps.
U.S. Pat. No. 5,095,126 and U.S. application Ser. No. 07/847,295, now U.S. Pat. No. 5,298,528, relate to the preparation of optically pure stereogenically labile 4-substituted-2-hydroxytetronic acid compounds.